You are on the right track

using the select-prev-word-loop snippet. After the selection loop, all you need to do is copy the selection, move the cursor to the right to unselect the selection and finally add the selection to the current glossary:

Note that once you have selected the words you want to add, you need to hit your resume hotkey (by default F8) in order to exit the selection loop and trigger the copy of the selection, the unselection of the selected text and finally the opening of the Add Glossary Entry window.